Converging evidence from both animals and humans indicates that high frequency gamma band (>30 Hz) activity is an important electrophysiological signature of information processing in the brain. Gamma activity is hypothesized to reflect neuronal synchronization during regional cortical processing and during the transmission and integration of information across cortical regions. Electrocorticographic (ECoG) recordings from subdural electrodes implanted for clinical purposes, provide an unusual opportunity to study this activity in humans in greater detail. The first 2 hypotheses to be tested are (1) that cortical processing within functionally specialized cortical networks is associated with ECoG gamma activity, and (2) that top-down modulation of cortical processing by selective attention is reflected by an increase of ECoG gamma activity. In addition, it is hypothesized that functional interactions between cortical regions engaged by a cognitive task are reflected by time-and frequency-dependent interactions between their ECoG signals, particularly in the gamma band. Finally, it is hypothesized that the aforementioned ECoG gamma responses, in combination with other ECoG indices of cortical activation, can be used to map cortical function prior to surgery. Specific aims: (1) Elucidate the relationships among phase-locked and non-phase-locked responses in gamma and other frequencies during activation of functionally specialized cortical regions. (2) Test the effect of selective attention on regional gamma responses during top-down modulation of cortical stimulus processing. (3) Test relationships between gamma responses and frequency-dependent estimates of signal transmission during functional interactions between cortical networks. (4) Test the utility of ECoG for functional mapping by comparing it with clinical electrocortical stimulation mapping. Significance: Basic knowledge derived from this research is expected to facilitate future investigations of the neurophysiological correlates of human brain function, as well as the development of new tools for functional brain mapping to minimize clinical impairment following resective brain surgery.